JP-A-2004-271756 (corresponding to US 2004/0173862 A1) describes a technique for integrating optical devices, such as a microlens or a microprism, made of SiO2 on a Si substrate.
In JP-A-2004-271756, in order to form an optical device, a Si substrate 201 is provided with both masks 207 for digging the outline of the optical device in the Si substrate 201 and for digging a plurality of trenches 208 for filling the inside of the optical device with SiO2, as shown in FIGS. 40A to 40C. Etching is performed to the Si substrate 201 with the masks 207 provided thereon to form columnar Si structures 203 having the outline in the same shape as that of the outline of the optical device and including the plurality of trenches 208.
Furthermore, in the states shown in FIGS. 40A to 40C, thermal oxidation treatment is applied to the columnar Si structures 203, thereby filling the trenches 208 with the SiO2 expanded by the thermal oxidation. The relationship between the width of the columnar Si structure 203 sandwiched between the adjacent trenches 208, and the width of the trench 208 is set such that the trench 208 is filled when the columnar Si structures 203 are thermally oxidized to the SiO2 structure.
However, the edge of the substrate 201 cannot be accurately formed by etching. For example, the corner of the trench 208 is formed in a smooth corner shape as shown in FIG. 41A. In the columnar Si structure 203 sandwiched between the two trenches 208, the position of a Si atom located at the same distance from both the trenches 208 is indicated by a point “a”, and the point of intersection between a Si structure forming the outline and the columnar Si structure 203 is indicated by a point “b”. When Ra is the shortest distance between the point “a” and the trench 208, and Rb is the shortest distance between the point “b” and the trench 208, the following relation is obtained: Ra<Rb. The shortest distance to the trench 208 depends on the point in the Si structure 203. The longer the distance of the Si atom to the trench 208, the more the time needed for the oxidation. Thus, when the Si atom at the point “a” is completely oxidized, the Si atom at the point “b” is not oxidized yet as shown in FIG. 41B. For this reason, in order to oxidize the Si atom at the point “b”, a part of the trench 208 needs to remain as some clearance for passage of oxygen molecules for oxidizing the Si atom at the point “b” at the time of the completion of oxidation of the Si atom at the point “a”. Moreover, this clearance will never be filled completely after the Si atom at the point “a” is oxidized, as shown in FIG. 41C. Conversely, the clearance extends in the arrangement direction of the columnar Si structures by the oxidation of the Si atom at the point “b”. For example, the Si atom located at the point “a” moves to a point “a′”, and the Si atom located at the point “b” moves to a point “b′”. In use of the optical device, the presence of the large clearance inside the device causes the transmitted input light to be refracted in an undesired direction, which does not emit the sufficient transmitted light. Accordingly, light transmission property of the optical device is reduced.